The Thick-billed Murre is very similar to the Common Murre. In breeding plumage the Thick-billed Murre is blacker above with a white point intruding into the black throat. In winter, the Thick-billed Murre has a darker face without the dark line on the cheek. At close range in all plumages, it has a thin, white line on the bill that the Common Murre lacks and an overall heavier bill. Razorbills have heavier bills, whiter flanks, and pointed tails. Guillemots also have thin bills but are all black with white wing patches in summer and are much paler in winter with white wing patches.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.